1. Field of the Invention
The present invention relates to a thin film transistor (TFT)-liquid crystal display (LCD) panel driving circuit capable of preventing stripes from occurring on a TFT-LCD panel.
2. Description of the Related Art
TFT-LCD display devices are widely used in laptops and computer monitors. Circuits for driving TFT-LCD panels may be generally divided into gate driver circuits and source driver circuits.
FIG. 1 is a circuit diagram of a prior art TFT-LCD device. Referring to FIG. 1, a typical TFT-LCD device 100 includes a liquid crystal panel 105, a source driver circuit 110, and a gate driver circuit 120.
Each pixel 150 of the liquid crystal panel 105 is comprised of a liquid crystal capacitor C1 and a switch T1. The number of pixels 150 in a row direction of the liquid crystal panel 105 is equal to a given number (L) of source lines, and the number of pixels 150 in a column direction of the liquid crystal panel 105 is equal to a given number (M) of gate lines.
In each pixel 150, one terminal of a liquid crystal capacitor C1 is connected to a switch T1. The switch T1 is configured as a metal oxide semiconductor (MOS) transistor, and the gate of the switch T1 is connected to a gate line 140 extending from the gate driver circuit 120. The gate driver circuit 120 turns switch T1 on and off in each of the pixels 150.
The source driver circuit 110 inputs a grey scale voltage to the liquid crystal panel 105 via a source line 130. The amount of the input grey scale voltage depends on input data. In other words, when switches connected to the gate line 140 are turned on by an output voltage of the gate driver circuit 120, a grey scale voltage output from the source driver 110 is applied to liquid crystal capacitors C1 connected to the switches.
The source driver circuit 110 includes a plurality of amplifiers (not shown) arranged at an output port. Since a random direct current (DC) offset exists in each of the amplifiers, the output voltages from each of the amplifiers are different, even when a grey scale voltage for the same input data has been selected and applied to the amplifiers.
This difference between output voltages output from the respective amplifiers of the source driver circuit 110 may cause a ‘stripe phenomenon’ on an LCD screen or display. The stripe phenomenon results in degradation scale in the quality of picture images displayed on the LCD screen.
A method for removing DC offsets in the amplifiers of the source driver circuit 110 has been disclosed in U.S. Pat. No. 6,331,846. The method in the '846 patent describes a prior art chopping process for averaging DC offsets by switching input ports of amplifiers.
The prior art chopping process of the '846 patent is described in the following paragraphs with reference to FIG. 2, which is a diagram illustrating a method of driving a pixel by alternately applying a positive polarity voltage and a negative polarity voltage to frames of a single pixel. Each liquid crystal pixel may be described in terms of one or more frames, i.e., a single pixel may include a plurality of frames. Here, a positive polarity voltage is a voltage greater than a common voltage (Vc in FIG. 1) applied to a liquid crystal panel by a source driver. A negative polarity voltage is a voltage smaller than the common voltage applied to the liquid crystal panel by the source driver. In order to extend the life span of a liquid crystal panel, driving voltages having opposite polarities are applied to liquid crystal pixels.
Referring to FIG. 2, in a first frame, even though a positive polarity voltage 211 is supposed to be output, a voltage of 212 is actually output, due to the existence of an offset voltage (hereafter ‘offset’) of +A. Likewise, in a second frame, even though a negative polarity voltage of 221 is supposed to be output, a voltage of 222 is actually output due to the existence of an offset +B. In order to cancel the offset +A, it is necessary to apply a positive polarity voltage in a third frame having an offset −A. In order to cancel the offset +B, it is necessary to apply a negative polarity voltage in a fourth frame having an offset −B.
However, a driving circuit taught by the '846 patent implements a chopping process by counting a clock signal activated on each gate line, so that a DC offset can increase or decrease in each frame. However, the frequency of a clock signal varies, depending on the resolution of a liquid crystal panel, and a clock signal is generated in a blanking period between an end point of one frame and a start point of a following frame, i.e., between two adjacent frames.
Accordingly, in the prior art liquid crystal display panel having a specific resolution, where offsets in voltages output from amplifiers of a source driver circuit are controlled by the frequency of a clock signal, i.e., how frequently a clock signal is activated, DC offsets in output voltages may accumulate rather than cancel each other out. In such a case, the stripe phenomenon is more likely to occur on an LCD screen.
FIGS. 3A and 3B are diagrams illustrating canceling and accumulation of DC offsets when driving a liquid crystal display panel using a conventional source driver circuit. FIG. 3A shows two different cases using the same number of gate lines of a liquid crystal display panel, but having different numbers of clock signals (CLK1) generated during a blanking period. In FIG. 3A, (1) illustrates that a DC offset in a first frame cancels out a DC offset in a second frame, and (2) shows that DC offsets in the first and second frames are accumulated.
FIG. 3B also shows two different cases having different numbers of liquid crystal display panel gate lines, i.e., different resolutions. In FIG. 3B, (1) illustrates that a DC offset in a first frame cancels out a DC offset in a second frame, and (2) shows that DC offsets in the first and second frames accumulate rather than cancel each other out.
As described above, since resolution of a prior art liquid crystal display panel or the frequency of occurrence of a clock signal during a blanking period in the LCD panel varies, DC offsets of a panel driving voltage applied to a pixel may accumulate rather than cancel each other out. Thus, the quality of picture images displayed on an LCD screen may deteriorate.